feat: Java SDK feature flags

[miguelhrocha]

Changes

This PR is the documentation task assuming that the following PRs get merged:

1. feat: feature flag polling for local evaluation (1/4) posthog-java#54
2. feat: integrate feature flag poller to PostHog client (2/4) posthog-java#55

NOTE: this PR needs an update after the implementation of:

• feat: get all feature flags related to a distinct_id (4/4) posthog-java#57

I will take care of it once the pull requests are merged.

It follows the pattern of the other SDKs by:

• Adding snippets on how to use the posthog client for feature flag evaluation
• Adding a section for feature flag variant retrieval

Additional changes

• Now that the Java SDK has the concept of "personalApiKey", the PostHog client initialization snippet change to reflect that.

Screenshots

• The PostHog style guide
• Full list of tags and categories
• List of content components
• PostHog SEO best practices

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